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Ditch Basics

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					                                            Ditch Basics
                                By Marisa DiBiaso, Project Assistant
    Water is the most destructive force in roads.      •   Triangular--The v-shaped bottom ditch is the
Ditches are the best way to carry water away from          most easily constructed and requires the least
roads and to the water table. Ditches catch sedi-          roadside area. Of the three types, it requires
ment carried off the road surface. This makes it           more maintenance, has the lowest water-car-
imperative that road managers keep ditches in              rying capacity, and is the most susceptible to
good condition. This article will cover the basics         erosion.
of good ditches: inspection, shape, slope, lining,
vegetation, and maintenance and repair.                Slope

Inspection                                                  The lengthwise slope affects the velocity of
                                                       flow. If the slope is too steep, the swift water flow
     The first step to ditch basics is to regularly    causes erosion and maintenance problems. If the
clean and inspect ditches to keep them working         slope is too flat, water stays in the ditch and can
properly. Use the following checklist to help          infiltrate the road base. Water in the base deterio-
identify needed maintenance.                           rates the road from within. To ensure water flow,
o Is the ditch free of obstructions?                   the minimum desired lengthwise slope is 1.0%. It
o Does the ditch have a clear outlet?                  should never be less than 0.5%. The maximum for
o Is the ditch deep enough to drain the subgrade       an unlined ditch is 5%. If the slope exceeds 5%,
    and cut off subsurface water?                      the ditch should be lined as described below to
o Is the ditch broad enough?                           prevent erosion.
o Is the slope uniform; that is, free of high or            Ditches with earth sides should have side
    low points to minimize ponding?                    slopes from 1:4 or 1:2 (drop to run). To calculate
o Could debris control devices be used?                slopes, see page 9 for a copy of "Measuring and
o Is there any erosion?                                Calculating Slopes."
o Is a lining needed/does the existing lining need
    repair?                                            Lining
o Is the slope adequate?
                                                            Erosion creates sediment that is deposited in
Shapes                                                 ditches and water bodies. The greater the erosion,
                                                       the more maintenance needed. Excessive erosion
     Highway crews can construct and maintain          can weaken the sides of the road itself.
ditches in three cross-section shapes: parabolic            Ditch lining is the most common way to pre-
(round bottom), trapezoidal (flat bottom), or trian-   vent erosion. Linings can be of natural soil, vege-
gular (v-shaped bottom).                               tative, or paved. The material used depends on
• Parabolic--This ditch is best in terms of long-      flow velocity. The following table illustrates the
    term cost and efficiency. It has about the same    types of lining appropriate for various flow
    capacity as the trapezoidal with less erosion.     velocities.
    Sides are easily vegetated, further reducing            Engineers use similar tables when designing
    erosion. It is probably the most difficult and     waterways. Road managers can also use it empiri-
    expensive to construct.                            cally. If a lined, well-maintained ditch is eroding,
• Trapezoidal--The flat bottom is easier to con-       then crews should apply a lining for a higher flow.
    struct than the round bottom (parabolic)' ditch.   If problems persist, the agency should consult a
    Compared to the triangular shape, the flat         qualified engineer.
    bottom slows water and reduces erosion. It
    takes more time and expense to construct, but
    requires less maintenance and greater capacity.
Road Business, Winter 2000, Vol. 15, No.4
                                                  page 4
                                                    Maximum            Periodically maintain all ditches. Schedule
                      Type of Lining                  Flow        routine maintenance" every five years. Poorly
itA.'
                                                    Velocity      shaped, sloped, or lined ditches require mainte-

'"        Natural Soil Linings
          Rip-rap sides and bottoms
          Cleangravel
                                                     (ft/sec)

                                                      15-18
                                                       6-7
                                                                  nance more often. To maintain ditches in a five-
                                                                  year cycle, divide municipal roads into five areas.
                                                                  Plan and schedule work on each section in a given
                                                                  year.
          Siltygravel                                  2-5
          Cleansand                                    1-2             Planning begins with inspection. Inspection in
          Silty sand, clay                             2-3        the fall enables a clear view of ditch conditions,
          Clayeysand, silt                             3-4        and allows planning over the winter. Planned
          Vegetative Linings                                      maintenance should produce ditches with adequate
          Average turf, erosion resistant soil        4-5         shape, slope, and linings. The following guidelines
          Average turf, easily eroded soil            3-4         should be considered:
          Dense turf, erosion resistant soil          6-8         • The most efficient and effective ditches have
          Gravel bottom, brushy sides                 4-5             flat or round bottoms. A backhoe or excavator
          Dense weeds                                 5-6             most easily constructs trapezoidal or parabolic
          Paved Linings                                               shaped ditches. If using a grader, run the wheel
          Gravel bottom, concrete sides               8-10            the ditch bottom.
          Mortared rip-rap                            8-10
                                                                  • Compact the ditch bottom.
          Concrete or Asphalt                         18-20
                                                                  • The lengthwise and side slopes should be uni-
        Vegetation                                                    form.
                                                                  • Immediately seed, mulch, and use fiber mats
             In addition to minimizing erosion in low-flow            to establish vegetation, or immediately apply
        ditches, vegetation removes roadway pollutants                other necessary lining.
        from runoff water. Here are a few guidelines to                Ditches should be an appropriate depth. To
        remember about vegetation in ditches.                     keep water out of the base material, the ditch bot-
        • Vegetative linings should be established                tom should be below the base course. A depth of
            BEFORE erosion begins.                                18 inches is usually sufficient, but it may have to
        • Seed, mulch, and, where necessary, fiber mats           be deeper if water flows in from hillsides. The
            should be applied immediately after any ditch         distance between cross culverts or ditch outlets
            maintenance or storm damage.                          influence depth. Inspection will indicate if depth is
        • Fertilizer often speeds growth, but excessive           adequate, or 'if culverts or outlets should be
            fertilization can cause groundwater and sur-          installed.
            face water quality problems.                               Ditches are often constructed too deep. Lining
        • Mowing prevents weeds and woody vegeta-                 is more difficult, and the added depth increases
            tion. Mowing too close will reduce the erosion        ditch erosion. Deep ditches can create a safety
            resistance ofthe lining.                              hazard to motorists, especially on roads with nar-
                                                                  row shoulders.
        Maintenance and Repair                                         Ditches are necessary to carry water off and
                                                                  away from roads and must be regularly main-
             Ditch repair should be included in all road
                                                                  tained. They must have the proper shape, slope,
        repair projects. Road rehabilitation and recon-
                                                                  and lining. Water flow, road surface sediment, and
        struction are expensive and some municipalities
                                                                  growing vegetation attack these features.
        omit ditch repair to save money. Some ignore
        ditches in minor projects, too. Inadequate ditches        Sources:

        will allow water to infiltrate the road base. This        Basics of a Good Road. UNH Technology Transfer Center. Durham, NH.
                                                                    JuneZOOO.
        creates inadequate drainage resulting in faster road      "Ditch/Channel Construction & Maintenance." A Series of Quick Guides for
        deterioration than if the time and money was spent          New Hampshire Towns. UNH Technology Transfer Center. Durham, NH.
                                                                  Drainage,Drainage, Drainage. UNH Technology Transfer Center. Durham
        on proper ditching.                                         NH. January 1996.

        Road Business, Winter 2000, Vol. 15, No.4
                                                              page 5

				
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